A portable apparatus is usually powered from a battery. The battery delivers a battery voltage to a circuitry of the portable apparatus. However, sometimes the circuitry requires a larger voltage than the battery voltage in order to operate. Moreover, the larger voltage is typically also required to be a substantially stable pre-determined voltage, e.g. with a predetermined voltage level of 5.5 V. For this purpose, a DC/DC converter is used to convert an input voltage, such as the mentioned battery voltage, to an output voltage, such as the pre-determined voltage, provided at an output terminal.
One type of DC/DC-converter is a so-called charge-pump type. A charge-pump type DC/DC converter is operated by charging a capacitor during a charging phase, by connecting the capacitor between the input voltage and a ground voltage during a charging period, followed by a discharging of the capacitor during a discharging phase, by connecting the capacitor between an output terminal and the input voltage during a discharging period, thus providing an output voltage at the output terminal. The output voltage may, in an idealized situation without e.g. any switching losses and parasitic losses, correspond to twice the input voltage, as the input voltage loaded onto the capacitor during the charging phase is thus added to the input voltage during the discharging phase. The ratio between the output voltage and the input voltage may be referred to as a gain factor. In practice, the converter will have some losses and the ratio between the output voltage and the input voltage will be limited to a gain factor which is somewhat less than two.
Charging and discharging the capacitor is typically performed using a plurality of switches, arranged to selectively connect one electrode of the capacitor to the input terminal or ground and to selectively connect the other electrode of the capacitor to the output terminal or the input terminal. The output voltage may be adjusted by adjusting a voltage drop over the switches, e.g. by adjusting the on-resistance of a transistor when a transistor is used as a switch. This however reduces the efficiency of the converter, as the voltage drop over the switches corresponds to additional power loss. This reduced efficiency has a significant impact on power consumption which is an important point of attention in mobile applications. The ratio between the adjusted output voltage and the input voltage may be referred to as a boosting factor. The boosting factor thus corresponds to the gain factor multiplied by the efficiency of the converter. When using the above described charge-pump type DC/DC-converter, a lower boosting factor than the gain factor may be achieved by reducing the efficiency. When the gain factor is two and the required boosting factor is 1.5, the efficiency of the converter has thus to be reduced to 75% (ignoring further losses).
European patent publication No. EP 1 073 185 A2 describes a charge-pump type DC/DC converter using two capacitors C1 and C2, each arranged to be chargeable and dischargeable using four switches for each of the capacitors, i.e. switches S1, S2, S3 and S4 for the capacitor C1 and switches S5, S6, S7 and S8 for the capacitor C2, as shown in FIG. 1. The four switches associated with one capacitor are capable of selectively connecting the corresponding capacitor between an input voltage Vin and a ground voltage GND during the charging period, followed by the discharging of the corresponding capacitor during the discharging phase, by connecting the corresponding capacitor between an output voltage Vout and the input voltage Vin during the discharging period. The capacitors C1 and C2 are arranged to be connected either in series or in parallel using a further switch S9, which allows by proper settings of the switches and the further switch to operate the converter in two different modes: a first mode with the two capacitors being connected in series, as shown in FIG. 2A, and a second mode with the two capacitors being connected in parallel, as shown in FIG. 2B.
In more detail, as shown in FIG. 1, the DC/DC converter is arranged to convert an input voltage Vin to an output voltage Vout, both measured relative to a reference voltage GND, typically ground. The first charge pump capacitor C1 has a first electrode C1a and a second electrode C1b and the second charge pump capacitor C2 has a first electrode C2a and a second electrode C2b. First switches S1, S5 associated with each of the charge pump capacitors can connect the second electrode C1b, C2b of the corresponding charge pump capacitor C1, C2 to the input voltage Vin. Second switches S2, S6 associated with each of the charge pump capacitors can connect the second electrode C1b, C2b of the corresponding charge pump capacitor C1, C2 to the reference voltage GND. Third switches S3, S7 associated with each of the charge pump capacitors can connect the first electrode C1a, C2a of the corresponding charge pump capacitor C1, C2 to the input voltage Vin. Fourth switches S4, S8 associated with each of the charge pump capacitors can connect the first electrode C1a, C2a of the corresponding charge pump capacitor C1, C2 to the output voltage Vout. The further switch S9 is provided between the first electrode C1a of the first charge pump capacitor C1 and the second electrode C2b of the second charge pump capacitor C2. The further switch S9 can thus connect the first charge pump capacitor C1 and the second charge pump capacitor C2 in series.
In a charge phase of a first mode of the DC/DC converter, as illustrated in FIG. 2A, the switches S1˜S9 establish a series arrangement of the first charge pump capacitor C1 and the second charge pump capacitor C2 in the charge phase between the input voltage Vin and the reference voltage GND, thus charging the first charge pump capacitor C1 and the second charge pump capacitor C2 to VC1=VC2=Vin/2, wherein VC1 denotes a voltage over the first charge pump capacitor C1 and VC2 denotes a voltage over the second charge pump capacitor C2. On the other hand, in a charge phase of a second mode of the DC/DC converter, as illustrated in FIG. 2B, the switches S1˜S9 establish a parallel arrangement of the first charge pump capacitor C1 and the second charge pump capacitor C2 in the charge phase between the input voltage Vin and the reference voltage GND, thus charging both the first charge pump capacitor C1 and the second charge pump capacitor C2 to VC1=VC2=Vin.
As described in EP 1 073 185 A2, the first and second charge pump capacitors C1, C2 may thus be loaded to either Vin/2 or Vin in the charge phase, using either the series arrangement of the first mode of FIG. 2A or using the parallel arrangement of the second mode of FIG. 2B respectively. Afterwards, a discharge phase of the DC/DC converter, as illustrated in FIG. 2C, is performed, wherein the switches S1˜S9 establish a parallel arrangement of the first charge pump capacitor C1 and the second charge pump capacitor C2 in a discharge phase between the input voltage Vin and the output voltage Vout, thus providing the output voltage as Vout=Vin+VC1=Vin+VC2, with equal VC1 and VC2. The DC/DC converter may thus provide an output voltage of Vout=2*Vin when the first charge pump capacitor C1 and the second charge pump capacitor C2 were loaded to Vin using the second mode in the charge phase prior to the discharge phase, whereas the DC/DC converter may provide an output voltage of Vout=1.5*Vin when the first charge pump capacitor C1 and the second charge pump capacitor C2 were loaded to VC1=VC2=Vin/2 using the first mode in the charge phase prior to the discharge phase. The DC/DC converter may thus be operated with a gain factor of 2 or 1.5.
As mentioned above, EP 1 073 185 A2 describes that when charging and discharging using the second mode with the two capacitors being connected in parallel, the gain factor is two (ignoring losses), whereas when charging using the first mode with the two capacitors being connected in series and discharging using the second mode with the two capacitors being connected in parallel, the gain factor is 1.5 (ignoring losses). Hence, two boosting factors, 2.0 and 1.5, can be provided at the maximum efficiency, by selecting the second mode for discharging after charging with the first or the second mode. It may be appreciated that, use of the circuit described in EP 1 073 185 A2, also allows to provide a boosting factor of three at maximum efficiency, by charging using the second mode with the two capacitors being connected in parallel and discharging using the first mode with the two capacitors being connected in series, as illustrated in FIG. 2D, associated with a gain factor of three.
The charge-pump type DC/DC converter described in EP 1 073 185 A2 thus allows to provide a plurality of modes with corresponding gain factors, allowing to operate the converter with maximum efficiency for a plurality of boosting factors. However, maximum efficiency can only be achieved at three different boosting factors (ignoring losses) using such charging in either series or parallel connection and discharging in either series or parallel connection. Even when increasing the number of capacitors to a larger value, e.g. N capacitors, which can all be connected either in series or in parallel, the maximum efficiency can only be achieved at three different boosting factors (ignoring losses), corresponding to a first gain factor of 2, a second gain factor of 1+1/N, and a third gain factor of 1+N.